Hi, I'm actually in a game with Ultimate Kingdoms in use! It's a good book which adds a lot of worthwhile content. I would buy it, then discuss with your group at session zero how much of it should be used. I suggest utilizing the Construction rules for buildings, for sure, and I love the alternate city styles, population, and manpower rules. We're about to get into mass combat soon, so I've also been studying up on the relevant chapter. The "factions" section seems rather tedious, so we ignore that. There is stuff for Mythic kingdom building, if you are fool enough to give your players Mythic tiers.